The acquisition of video image sequences is still today largely carried out in analogue form so that the images, once acquired and possibly transmitted and stored in analogue formats, exhibit an appreciable share of noise in their content. Once digitized, these images are also often subjected to storage/editing operations which, in their turn, introduce noise, this time of a digital nature. Finally, an image sequence generally undergoes a succession of transformations resulting in spatio-temporal noise of a highly random nature.
To obtain high-performance operation, the noise reduction methods call upon recursive filtering which considers the very high temporal correlation of the images of a video sequence. These filters are very effective but have the drawback of making the noise homogeneous and hence of giving the final result a “dirty window pane” effect. This is especially visible in the dark regions of the image.
The invention therefore relates to the homogenization of the dark regions of an image based on the amplitude of the luminance of the pixels of this image.
U.S. Pat. No. 4,962,426 filed in the name of the company Hitachi Limited discloses noise reducing systems which use the amplitude of the luminance. Such systems use the amplitude of the luminance to increase the stringency of the noise reducing filter, in the analogue domain.
Such systems are therefore not adapted for homogenizing the dark regions of an image having undergone a deterioration as they pass through a recursive filter. Moreover, such systems do not allow variable smoothing of an image and are not adapted to operate in the domain of digital images.